The invention relates to a bone cement system having a mixing facility for the mixing and dispensing of bone cement, a reservoir container for a monomer, and a conveyor, wherein the mixing facility comprises a mixing cylinder, the mixing cylinder stores a bone cement powder, the monomer can be conveyed from the reservoir container into the mixing cylinder by the conveyor, a sieve element is arranged between the reservoir container and the mixing facility to prevent the ingress of the bone cement powder from the mixing cylinder into the conveyor, and the mixing facility comprises a dispensing opening for dispensing a bone cement obtained by mixing the bone cement powder and the monomer.
PMMA bone cements have been known for decades and are based on the groundbreaking work of Sir Charnley (Charnley, J., “Anchorage of the femoral head prosthesis of the shaft of the femur,” J. Bone Joint Surg. 42: 28-30 (1960)). The basic structure of PMMA bone cements has remained the same ever since. PMMA bone cements includes a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator (N,N-dimethyl-p-toluidine) dissolved therein. The powder component comprises one or more polymers made by polymerization, preferably suspension polymerization, based on methylmethacrylate and co-monomers, such as styrene, methylacrylate or similar monomers, a radio-opaquer, and the initiator, dibenzoylperoxide. When mixing the powder component with the monomer component, swelling of the polymers of the powder component in the methylmethacrylate leads to the formation of a dough that can be shaped plastically. When mixing the powder component with the monomer component, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide while forming radicals. The radicals thus formed trigger the radical polymerization of the methylmethacrylate. Upon advancing polymerization of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies.
Polymethylmethacrylate bone cements can be mixed in suitable mixing beakers by spatulas by mixing the cement powder with the monomer liquid. This procedure is disadvantageous in that inclusions of air may be present in the cement dough thus formed and may later cause destabilization of the bone cement—also referred to as cement. For this reason, it is preferable to mix bone cement powder with the monomer liquid in vacuum mixing systems, since mixing in a vacuum almost completely removes inclusions of air from the cement dough and thus attains optimal cement quality (Breusch, S. J. et al., “Der Stand der Zementiertechnik bei Hiifttotalendoprothesen in Deutschland” [Current Status of Cemented Total Hip Arthroplasty in Germany], Z Orthop., 137: 101-07 (1999)). Bone cements mixed in a vacuum have substantially lower porosity and thus show improved mechanical properties. A large number of vacuum cementing systems have been disclosed of which the following shall be named for exemplary purposes: U.S. Pat. Nos. 6,033,105, 5,624,184, 4,671,263, 4,973,168, 5,100,241, 5,586,821, and 5,344,232, International Patent Application Publication Nos. WO99/67015 A1 and WO94/26403 A1, European patent application Publication Nos. EP 1 020 167 A2, EP 1 016 452 A2, EP 0 692 229 A1, and EP 1 005 901 A2, and German published patent application No. DE 36 40 279 A1.
A further development is cementing systems in which both the cement powder and the monomer liquid are already packed in separate compartments of the mixing systems and are mixed with each other in the cementing system only right before application of the cement (U.S. Pat. Nos. 5,997,544 and 6,709,149 and EP 0 692 229 A1). A drawback of all of these systems is the transfer of the monomer liquid into the cement powder and the complete mixing of these two components to obtain a homogeneous cement dough which must, in particular, not contain any regions of cement powder that has not been wetted by the monomer liquid. In a mixing system that is currently on the market in Europe, tubes that are arranged on the side of the lower part of the cartridge and penetrate through the cartridge wall are used to introduce the monomer liquid approximately into the middle of the cement powder through the application of a vacuum. There is no mixing facility provided at the tubes that might prevent the ingress of cement powder into the tubes during storage of the mixing system. Clogging of the tubes by cement powder cannot be excluded completely.
Another mixing system for the mixing and dispensing of bone cement is shown in German Patent document DE 698 12 726 T2. This mixing system comprises a mixing cylinder, whereby a sieve element is arranged between the reservoir container and the mixing facility to preventingress of the bone cement powder from the mixing cylinder into the conveyor. The mixing systems have proven to be disadvantageous in that homogeneous and rapid mixing of the monomer and the bone cement powder cannot be ensured at all times.
Another variant was disclosed in European Patent EP 1 140 234 B1. In this mixing system, the monomer liquid is aspirated through the entire cement powder by a vacuum. The basic approach, i.e., to aspirate the monomer liquid through the entire cement powder in order to achieve optimal mixing and prevent regions of non-wetted cement powder from forming is feasible only if a cement powder is used that swells very slowly after being wetted by the monomer liquid. This means that the high and medium viscosity PMMA bone cements, which currently are most commonly used in endoprosthetics, can be used not at all or with difficulties, since the cement powder of these cements swells immediately after being wetted by the monomer liquid and forms a dough that renders further pervasion of the cement powder by the monomer liquid difficult or even impossible.